The present invention relates to a method of forming a copper circuit pattern on a ceramic substrate. More particularly, the present invention relates to such a method in which the copper is bonded to the ceramic substrate by a direct bond process.
It is known in the prior art to bond copper directly to a ceramic substrate (such as aluminum oxide) and then to etch the copper to form a copper circuit pattern on the ceramic substrate. The copper is directly bonded to the substrate by forming a copper/copper oxide melt between the copper and the substrate. Upon the cooling of the melt, a strong bond is formed between the copper and the substrate. This process is fully described in U.S. Pat. No. 3,766,634.
After the copper is directly bonded to the substrate, the copper circuit pattern is formed by conventional techniques such as are known in the art for fabricating a printed circuit. A printed circuit is produced by a etching process in which the copper is etched in a pattern corresponding to the desired electrical pathways of the circuit. In forming the printed circuit, a mask of resist is applied to the copper by silk screen printing. The copper and substrate are then submerged in a bath of etching solution, generally an aqueous solution of hydrochloric acid. After the elapse of a suitable time interval, copper is removed by the hydrochloric acid in the unmasked areas down to the substrate to produce a copper circuit pattern that is identical to the pattern of the applied mask.
The thicker the copper circuit pattern, the more electrical load carrying capability of the finished circuit. The manufacture of thick copper circuit patterns in which the copper is directly bonded to the substrate has, however, proven to be uneconomical because of unacceptably long etching times involved in etching thick layers of copper. Additionally, well known and undesirable shadow effect is produced as a result of the prolonged exposure to acid. As will be discussed, the present invention provides a method of forming a copper circuit pattern on a ceramic substrate that incorporates the virtues of the strong bond that is effected between copper and a ceramic substrate by a direct bond process while making it economical to form copper conductor patterns of increased thickness in such directly bonded copper without the occurance of shadow effect.